Stabilization against liquefaction, for high bearing capacity, and reduced compressibility of foundation soil at depth are essential requirements to insure the stability of engineered structures built thereon. It is also essential to insure that no internal erosion under existing hydraulic seepage gradients and through permeable channels within the soil mass could lead to settlements and even to the development of sinkholes. These requirements are particularly important for large and/or sensitive structures such as bridges, dams, high-rise buildings and retaining structures, among others. It is also a major concern for slopes and stockpiles in general, and in particular when roads and railroads are built and used near them. It is a further concern for retaining structures of contaminated soils and mine tailings.
The properties of the foundation soil will have an important impact on the foundation's bearing capacity and its ability to withstand liquefaction. A vast area of the earth's surface is covered by loose sedimentary soil deposits which include thick strata of poorly graded soils that are prone to liquefaction during earthquakes, and which remain unstable even after deep densification. Generally, these soils do not yield an allowable bearing capacity above 150 kPa after densification and remain sensitive to liquefaction during earthquakes. As such, depending on a given location, the natural soil may not be suitable for supporting certain types of large and/or sensitive structures.
Several techniques exist to improve soil conditions so that the soil is more suitable for supporting structures. These techniques involve densifying the soil by using specialized tools and/or reinforcing the soil by embedding specialized structures therein. While these techniques have proven useful for some applications, there is much room for improvement.
Dynamic compaction increases soil density through repeated high energy impacts. This technique involves repeatedly dropping a heavy weight onto the ground at regular intervals. The force of impact of the weight causes the ground to compact and thus increase its bearing capacity. This technique is most effective for well-graded soil, and when densification at depths greater than 10 m is not required. Disadvantageously, the high energy impacts can cause undesirable effects to nearby structures, such as railroad tracks or buildings for example, due to vibration. Further, the existence at depth of undesirable soils or materials impact greatly on the efficiency of direct dynamic compaction. This is particularly true in case of sensitive clay formation or presence in the soil volume.
Vibroflotation, also referred to as vibro compaction, is another soil densification technique which increases soil density through vibration. This technique involves vibrating a cylindrically-shaped vibroflot or plunger in the ground, encouraging soil particles to rearrange in a more compact fashion. The vibration of the vibroflot induces an acceleration and vibration of the soil particles, allowing the vibroflot to be lowered into the ground. Once the soil is sufficiently compacted, the vibroflot is raised out of the ground. As with dynamic compaction, this technique works best on well-graded soil. Disadvantageously, this technique can be quite expensive, and is not effective when the soil is uniformly graded. Moreover, this technique leaves significant volumes of non-stabilized soils between the treated soil in the ground and cannot be performed where adjacent structures are close by.
Stone columns, also referred to as vibro replacement, is a technique for reinforcing and densifying soil. This technique involves creating a grid or lattice of stone columns underground by forcing stones of varying sizes into the soil. The columns act as reinforcements, providing discrete areas of increased rigidity in the soil which have an increased bearing capacity. Soil is also densified using this technique, as the action of forcing the stones into the soil causes soil surrounding the columns to be compacted. Disadvantageously, this technique is significantly more expensive relative to other techniques such as dynamic compaction. Also, this technique may cause the resulting soil to have inconsistent strength: uniform soil in the space between columns is weaker than the soil in and surrounding the columns. Uniform soil between columns is not transformed and may therefore still have undesirable properties. As a result, soil reinforced by this technique may not be well suited for withstanding earthquakes. During an earthquake, the uniform soil between columns can liquefy and displace, thus causing the columns to deform and/or break. Further undesirable mixing of the natural soils with the gravel of the stone columns often occurs and reduces the vertical permeability of the stone column and impairs its efficiency as a potential relief column for the pore pressures generated at depth, under a seismic event. The stone columns may also not always succeed to reach the bottom of the liquefiable layer which in the past has led to major damage during earthquake. Another concern occurs when the liquefied layer lies over a sensitive and or weak clay formation: the base of the stone columns would in this case rest on the weak layer. The load transfer from the stone column during an earthquake could become excessive should the confinement of the walls of the stone column become affected by the moving or by the settlement of liquefiable soils still present between the stone columns.
The cemented columns technique is a technique for reinforcing soil by creating a grid or lattice of cement-based columns underground. The technique involves drilling holes in the ground and filling the holes with a cement-based material. This technique is even more expensive than the stone columns technique. As with stone columns, the cemented columns technique may cause the resulting soil to have inconsistent strength: uniform soil between columns is not transformed, and may still have undesirable properties, making it susceptible to liquefaction. Cemented columns may therefore also not be well suited for withstanding earthquakes.
Another technique, known as engineered soils, involves replacing the natural soil entirely. If the natural soil has undesirable properties, for example if it cannot be sufficiently compacted, the soil can be excavated and replaced with a more suitable better graded soil. While this technique allows for a homogeneous strength of the resulting soil, it can be quite expensive and labor intensive as a large amount of soil will have to be transported to and from remote locations. Also, conventional compaction of saturated engineered fill may be problematic to achieve the desired degree of compaction by means of conventional compaction equipment.
Also known to the Applicant are the following publications: U.S. Pat. Nos. 6,802,805; 6,193,444; 6,000,641; 5,927,907; 5,199,196; 4,458,763; DE 19627465; DE 19612074; and EP 470297.
Despite these know techniques, there is a need for a method of soil treatment or transformation which, by virtue of its steps, design and/or components, would be able to overcome or at least minimize some of the aforementioned prior art problems.